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Man am I bored...........4 liters weighs 8.45 pounds.1 btu raises 1 pound of water 1 degree.assume the temp of the ice is at 0fto raise it to 103 will requires 870 btu'sfor 300 gallons of water (weighing 2490 pounds) at 103 degrees to warm the ice to 103 degrees the water will have to "give up" 870 btu'sThe spa will have to "give up" 2490 BTU's to drop in temp 1 degree.... from the above, 840 BTU's will raise the ice to 103. By my calculations, the 2 two liters of ice should drop the temp of a 300 gallon spa at 103 degrees by only 1/3 of a degree.